the baffle system you have in that drawing will just wind up being submerged peices of acryllic. You have to have one that comes down from the top of the tank to stop the water level from rising.
i have an inch hose thats about 3 inches long stemming from my OFB that connects to 3/4 inch CPVC piping. Apparently CPVC is used for drinking waters and meets certain requirements, plus its like 10 cents more expencive so i figured why not. any way 3/4 inch ID pipe to the sump allows for ~220 GPH and for my nano that's plenty. and i have 1/2 inch ID on my return pump.